Ambiguity compensation in time-of-flight ranging

ABSTRACT

Methods and apparatus are provided for measuring distance from an instrument origin to each of a plurality of points in an environment. Laser pulses are emitted along a measurement axis at successive displacements about the origin. The emission time of each pulse is time-shifted relative to a fixed rate. The time shift corresponds to an index of a repetitive sequential pattern. Received pulses are detected at respective arrival times. For each received pulse: a current apparent distance is determined, a measured delta distance is calculated, a range interval is assigned by comparing measured delta distance with an expected delta distance synchronized with the index of the latest emitted pulse, the current apparent distance is defined to be a true measured distance for any received pulse assigned to a first time interval, otherwise the current apparent distance is defined to be a false measured distance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/445,895, filed Jul. 29, 2014 which claims priority to French PatentApplication No. FR 13/02082, filed Sep. 9, 2013, each of which isincorporated herein by reference in their entirety for all purposes.

TECHNICAL FIELD

The present invention relates to the field of electronic distancemeasurement. More particularly, the present invention relates to methodsand apparatus for compensating ambiguity in time-of-flight ranging.

BACKGROUND ART

Ambiguity interval limitation is a classic issue for distancemeasurement techniques.

For distance measurement based on phase shift, the issue has beenaddressed by using different frequencies of amplitude modulation: alower frequency for resolving ambiguity, and a higher frequency foraccuracy.

In time-of-flight distance measurement, the delay between emission timeof an emitted laser pulse and detection time of a return pulse allowsfor distance calculation. When the measurement rate is high (e.g., 1MHz), the time between emitted pulses is short, (e.g., 1 μs), so thatthe return pulse from a first emitted pulse can return after a secondemitted pulse or even a third emitted pulse has been sent. Thisambiguity makes it unclear whether a return pulse corresponds to thefirst, second or third emitted pulse.

If distance is measured between the emission time of a given emittedpulsed and the detection time of a return pulse resulting from a formeremitted pulse, then the measured distance is diminished by a multiple ofthe ambiguity interval limit (e.g., −150 m for 1 μs of light travelingduration). So for example, a building at 193 m would be incorrectlymeasured at 43 m. Incorrectly measured distances result inthree-dimensional point measurements which interfere with other, correctmeasurements (e.g., when measurements are displayed as a point cloud),with no easy way to filter or separate the incorrect measurements fromthe correct measurements.

In aerial scanning, where the variation of measured distances is smallrelative to the measurement range, the issue can be addressed by timewindowing. See, for example, International Patent Publication WO2008/107129 A1 dated 12 Sep. 2008, and U.S. Pat. No. 8,212,998 B2 dated3 Jul. 2012. The system can expect a return pulse within a measurementwindow: a reasonable assumption for airborne LIDAR where the distance tobe measured is always between two limits given by the flight altitudeand the possible ground variations. This method is not suitable forterrestrial laser scanning where measured distances vary from theminimum to the maximum possible measurement range.

One solution for terrestrial scanning is to apply pulse signatures tothe emitted pulses. See for example International Patent Publication WO2009/039875 dated 2 Apr. 2009, and U.S. Pat. No. 8,149,391 dated 3 Apr.2012. Applying pulse signatures makes it possible to distinguish pulsesfrom one another. Recognizing the order of emitted pulses in a sequenceenables the return pulses to be sorted in correct order. Pulses can be“signed” in different ways, such as by emitting a doublet of pulses foreach distance measurement with the time (and thus the distance) betweenthe two pulses of the doublet varying according to a defined sequence.The signature is recognized as the time between a pair of receivedpulses. The pulse signature approach has the disadvantage ofimplementation complexities, such as generating doublets,differentiating doublets from one another, and recognizing the signaturefrom a noisy return doublet.

Additional ranging techniques are described in Patent ApplicationPublication US 2010/0128248 A1 dated 27 May 2010, Patent ApplicationPublication US 2011/0038442 A1 dated 17 Feb. 2011, European PatentApplication Publication EP 2 469 297 A1 dated 27 Jun. 2012, PatentApplication Publication US 2012/0257186 dated 11 Oct. 2013, and EuropeanPatent Application Publication EP 1 413 896 A2 dated 28 Apr. 2004.

Simple and effective techniques to address the ambiguity intervallimitation are desired, especially techniques useful for terrestrialscanning.

SUMMARY

Electronic distance measurement methods and apparatus enablingtime-of-flight ranging with ambiguity resolution are provided inaccordance with embodiments of the present invention. Distance ambiguityis compensated in time-of-flight measurement with a high measurementrate (e.g., of 1 MHz) or a medium measurement rate (e.g., of 400 kHz).In some embodiments, incorrect measurements made at a range above afirst ambiguity interval limit (e.g., −150 m for 1 MHz measurement rateor ˜340 m for 400 kHz measurement rate) are identified. Thesemeasurements are filtered out or corrected for the ambiguity.

Methods and apparatus are provided for measuring distance from aninstrument origin to each of a plurality of points in an environment.Laser pulses are emitted along a measurement axis at successivedisplacements about the origin. The emission time of each pulse istime-shifted relative to a fixed rate. The time shift corresponds to anindex of a repetitive sequential pattern. Received pulses are detectedat respective arrival times. For each received pulse: a current apparentdistance is determined, a measured delta distance is calculated, a rangeinterval is assigned by comparing measured delta distance with anexpected delta distance synchronized with the index of the latestemitted pulse, the current apparent distance is defined to be a truemeasured distance for any received pulse assigned to a first timeinterval, otherwise the current apparent distance is defined to be afalse measured distance.

Embodiments of the invention are based on a principle of scrambling thepoint cloud for distances above the ambiguity limit by having anirregular pulse repetition rate.

BRIEF DESCRIPTION OF DRAWING FIGURES

These and other aspects and features of the present invention will bemore readily understood from the embodiments described below withreference to the drawings, in which:

FIG. 1 schematically illustrates a system for implementing a distancemeasurement scheme in accordance with some embodiments of the invention;

FIG. 2 schematically illustrates elements of a system for implementing adistance measurement scheme in accordance with some embodiments of theinvention.

FIG. 3 schematically illustrates a system for implementing a distancemeasurement scheme in accordance with some embodiments of the invention;

FIG. 4A is a timeline of a distance-measurement scenario;

FIG. 4B is a timeline of a further distance-measurement scenario;

FIG. 4C is a timeline of a distance-measurement scenario in accordancewith some embodiments of the invention;

FIG. 4D is a timeline of a distance-measurement scenario in accordancewith some embodiments of the invention;

FIG. 5A illustrates a scenario in which the emission times ofmeasurement pulses are maintained at a fixed repetition rate;

FIG. 5B illustrates a scenario in which the emission times ofmeasurement pulses are scrambled in accordance with some embodiments ofthe invention;

FIG. 6A is a timeline showing pulses emitted periodically withoutscrambling;

FIG. 6B is a timeline showing pulses whose emission times are scrambledin accordance with some embodiments of the invention;

FIG. 6C is a table showing delta distance for each index in a secondinterval in accordance with some embodiments of the invention;

FIG. 6D is a table showing delta distance for each index in a thirdinterval in accordance with some embodiments of the invention;

FIG. 7 indicates how an interval range is obtained for each of aplurality of intervals in accordance with some embodiments of theinvention;

FIG. 8A is a timeline showing the result of measurements in a firstinterval, using pulses emitted periodically without scrambling;

FIG. 8B is a timeline showing the result of measurements in a firstinterval, using pulses whose emission times are scrambled in accordancewith some embodiments of the invention;

FIG. 9A is a timeline showing the result of measurements in a secondinterval, using pulses emitted periodically without scrambling;

FIG. 9B is a timeline showing the result of measurements in a secondinterval, using pulses whose emission times are scrambled in accordancewith some embodiments of the invention;

FIG. 10A is a timeline showing the result of measurements in a thirdinterval, using pulses emitted periodically without scrambling;

FIG. 10B is a timeline showing the result of measurements in a thirdinterval, using pulses whose emission times are scrambled in accordancewith some embodiments of the invention;

FIG. 11A schematically illustrates measurements in a second interval,using pulses emitted periodically without scrambling;

FIG. 11B schematically illustrates measurements in a second interval,using pulses whose emission times are scrambled in accordance with someembodiments of the invention;

FIG. 12 shows a method of identifying that measurements are in a secondinterval, in accordance with some embodiments of the invention;

FIG. 13 illustrates the measurement of distances with pulses whoseemission times are scrambled in accordance with some embodiments of theinvention;

FIG. 14 illustrates identification of measurement interval in accordancewith some embodiments of the invention;

FIG. 15 shows an example of interval ambiguity;

FIG. 16 shows an overview of point tagging in accordance with someembodiments of the invention;

FIG. 17 shows point-tagging rules in accordance with some embodiments ofthe invention;

FIG. 18A shows a scanning scenario;

FIG. 18B shows an example of point tagging in the scenario of FIG. 18Ain accordance with some embodiments of the invention;

FIG. 19A shows a scanning scenario;

FIG. 19B shows an example of point tagging in the scenario of FIG. 19Ain accordance with some embodiments of the invention;

FIG. 20A shows a scanning scenario;

FIG. 20B shows an example of point tagging in the scenario of FIG. 20Ain accordance with some embodiments of the invention;

FIG. 21 shows tag contagion in accordance with some embodiments of theinvention;

FIG. 22 is a flow chart of point tagging in accordance with someembodiments of the invention;

FIG. 23 describes extensions in accordance with some embodiments of theinvention;

FIG. 24 illustrates two-dimensional scrambling of the emission times ofmeasurement pulses in accordance with some embodiments of the invention;

FIG. 25 illustrates two-dimensional signature recognition in accordancewith some embodiments of the invention;

FIG. 26 describes two-dimensional interval assignment in accordance withsome embodiments of the invention;

FIG. 27 shows a method of measuring distance in accordance with someembodiments of the invention; and

FIG. 28 schematically shows a system for measuring distance inaccordance with some embodiments of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a scanner system 100 for implementing adistance measurement scheme in accordance with some embodiments of theinvention. System 100 includes a distance meter 105, one or moredeflection units 110, and a system control unit 115. Distance meter 105emits laser pulses along a path 120, which is deflected over a scanangle 125 by deflection unit/s 110. Distance meter 105 detects laserpulses returning from a scene or object 130 and determines a measureddistance based on time of flight of each pulse. System control unit 115and/or distance meter 105 can include one or more processors 135, 140programmed to carry out functions described herein. System 100 can alsohave a communications link 145 to enable exchange of data with anexternal processor (not shown) programmed to carry out functionsdescribed herein.

A scanner system used on a moving vehicle can have a single deflectionunit 110 to scan the laser path 120 about a vehicle path. A stationaryscanner system can have two deflection units 110, each scanning thelaser path about one of two orthogonal axes.

FIG. 2 schematically illustrates elements of a scanner system 200 forimplementing a distance measurement scheme in accordance with someembodiments of the invention. System 200 includes a distance meter 205,a fast deflection unit 210 rotating a mirror 215 about a first axis 220,a slow deflection unit rotating mirror 215 about a second axis 230, anda system control unit 235. Distance meter 205 emits laser pulses along apath 240, which is deflected over a scan angle 245 by deflection units210 and 225. Distance meter 205 detects laser pulses returning from ascene or object 250 and determines a measured distance based on time offlight of each pulse.

Axis 220 can lie substantially in a horizontal plane so that rotation ofmirror 215 about axis 220 scans the path 240 in a substantially verticalplane. Axis 230 can lie substantially in a vertical plane so thatrotation of mirror 215 about axis 230 rotates the path 240 azimuthally.Scanner 200 is suited for stationary scanning of a scene or object 250.The result of a scan is a cloud of measurement points in a sphericalcoordinate system centered on the intersection of axis 220 and axis 230.

System control unit 235 and/or distance meter 205 can include one ormore processors 255, 260 programmed to carry out functions describedherein. System 200 can also have a communications link 265 to enableexchange of data with an external processor (not shown) programmed tocarry out functions described herein.

FIG. 3 schematically illustrates at 300 a distance meter 305 forimplementing a distance measurement scheme in accordance with someembodiments of the invention. Distance meter 305 can be used as distancemeter 105 of system 100 or as distance meter 205 of system 200. Distancemeter 305 has a source 310 of laser pulses, an optical unit 315, areception unit 320, and a distance-meter control unit 325. Optical unit315 directs laser pulses from source 310 along path 330, and directsreturn pulses from scene or object 340 to reception unit 320. Receptionunit 320 includes, for example, an avalanche photodiode (APD), ahigh-speed pulse detector circuit (RTAC) and a digitizer. Distance meter305 can be as described, for example, in International PublicationNumber WO 2009/039875 A1 and U.S. Pat. No. 8,149,391 B2, which areincorporated herein by this reference.

FIG. 4A is a timeline of a distance-measurement scenario, in which eachreturn pulse (R1, R2, R3, . . . ) is detected by the distance meterbefore a new pulse is emitted (E2, E3, E4, . . . ). In this example,return pulse R1 corresponds to emitted pulse E1 and results in ameasured distance D1, return pulse R2 corresponds to emitted pulse E2and results in a measured distance D2, return pulse R3 corresponds toemitted pulse E3 and results in a measured distance D3, and return pulseR4 corresponds to emitted pulse E4 and results in a measured distanceD4. For a measurement rate of 1 MHz the period of pulse emission is 1μs, corresponding to a distance of about 150 m. In this example, alldistances (D1, D2, D3, D4) are measured correctly because they are lessthan 150 m. (While the example given here is for a measurement rate of 1MHz, other embodiments have a different measurement rate. For example, ameasurement rate of 400 kHz corresponds to a 2.5 μs period of pulseemission and a distance of about 375 m.)

FIG. 4B is a timeline of a further distance-measurement scenario, inwhich it is not known which return pulse (R1, R2, R3, . . . ) detectedby the distance meter corresponds to which emitted pulse (E2?, E3?, E4?,. . . ). In this example, return pulse R1 corresponds to emitted pulseE1, but because it is received after emission of emitted pulse E2 thereis an ambiguity as to whether it corresponds to emitted pulse E1 oremitted pulse E2; the measured distance will appear to be D2 (less than150 m) rather than the true distance (greater than 150 m). Similarly,return pulse R2 corresponds to emitted pulse E2, but because it isreceived after emission of emitted pulse E3 there is an ambiguity as towhether it corresponds to emitted pulse E2 or emitted pulse E3; themeasured distance will appear to be D3 (less than 150 m) rather than thetrue distance (greater than 150 m). Likewise, return pulse R3corresponds to emitted pulse E3, but because it is received afteremission of emitted pulse E4 there is an ambiguity as to whether itcorresponds to emitted pulse E3 or emitted pulse E4; the measureddistance will appear to be D4 (less than 150 m) rather than the truedistance (greater than 150 m). All distances (D1, D2, D3) are greaterthan 150 m and are measured incorrectly as being less than 150 m.However, measured distances D1, D2, D3 are diminished by the period ofpulse emission (1 μs, corresponding to a distance of about 150 m), andare still geometrically consistent with one another.

FIG. 4C is a timeline of a distance-measurement scenario in accordancewith some embodiments of the invention. In this example, pulse emissiontimes of the pulses are varied from a nominal 1 μs period (indicated bydotted vertical lines) such that pulse E2 is emitted 0.97 μs after pulseE1 (corresponding to about 145 m) and pulse E3 is emitted 1.03 μs afterpulse E2 (corresponding to about 155 m). In this example, return pulseR1 corresponds to emitted pulse E1 and results in a measured distanceD1, return pulse R2 corresponds to emitted pulse E2 and results in ameasured distance D2, return pulse R3 corresponds to emitted pulse E3and results in a measured distance D3, and return pulse R4 correspondsto emitted pulse E4 and results in a measured distance D4. In thisexample, all distances (D1, D2, D3, D4) are measured correctly becausein each case the return pulse arrives before emission of the nextemitted pulse, e.g., the measured distances are correct if are they areless than 145 m-155 m. The slightly irregular pulse emission rate doesnot affect the measured distances in this example.

FIG. 4D is a timeline of a distance-measurement scenario in accordancewith some embodiments of the invention. In this example (as in FIG. 4C),pulse emission times of the pulses are varied from a nominal 1 μs period(indicated by dotted vertical lines) such that pulse E2 is emitted 0.97μs after pulse E1 (corresponding to about 145 m) and pulse E3 is emitted1.03 μs after pulse E2 (corresponding to about 155 m). In this example,return pulse R1 resulting from emitted pulse E1 arrives after emissionof pulse E2, so that the apparent measured distance D2 is based onemitted pulse E2. Similarly, return pulse R2 resulting from emittedpulse E2 arrives after emission of pulse E3, so that the apparentmeasured distance D3 is based on emitted pulse E3, and return pulse R3resulting from emitted pulse E3 arrives after emission of pulse E4, sothat the apparent measured distance D4 is based on emitted pulse E4. Inthis example, measured distances D2, D3, D4 are incorrect (as in FIG.4B), except that D2 and D4 are diminished by 155 m while D3 isdiminished by 145 m. Unlike the example of FIG. 4B, measured distance D3is not geometrically consistent with measured distances D2 and D4.

FIG. 5A schematically illustrates a scenario 500 in which the emissiontimes of measurement pulses are maintained at a fixed repetition rate,as in the examples of FIG. 4A and FIG. 4B. A scanner system 505 scans ascene having a building 510 in a first interval of less than 150 m and abuilding 515 in a second interval of 150 m-300 m. Points 520 representcorrect measurements on the face of building 510. Points 525 representcorrect measurements on a ground surface between scanner system 505 andbuilding 510. Points 530 represent incorrect measurements of points onthe face of building 515, which appear as “ghost” points at a range ofless than 150 m because of the ambiguity illustrated in FIG. 4B.

FIG. 5B schematically illustrates a scenario in which the emission timesof measurement pulses are varied (scrambled), as in the examples of FIG.4C and FIG. 4D. In this example, scanner system 505 scans a scene havinga building 510 in a first interval of less than 150 m and a building 515in a second interval of 150 m-300 m. Points 520 represent correctmeasurements on the face of building 510. Points 525 represent correctmeasurements on a ground surface between scanner system 505 and building510. Points 555 and points 560 represent incorrect measurements ofpoints on the face of building 515, which appear as “ghost” points at arange of less than 150 m because of the ambiguity illustrated in FIG.4D. In this example, points 555 and 560 are staggered (rather thanvertically aligned as points 530) due to the variation of pulse emissiontimes illustrated in FIG. 4D.

Points measured at a range above the ambiguity interval are not easy toidentify (and thus to remove) because they look like normal points.

Some embodiments of the invention make the “ghost” points (incorrectlymeasured points) look like parasitic points and thereby allow them to beidentified and removed from the point cloud. By applying a smallvariation in the pulse repetition rate from one pulse to another,measured distances below the ambiguity interval limit are not affected,but measured distances above the ambiguity interval limit have a varyingbias.

FIG. 6A is a timeline showing pulses emitted periodically withoutscrambling. In this example, pulses are emitted with a fixed 1 μs periodat times 605, 610, . . . , 630.

FIG. 6B is a timeline showing pulses whose emission times are scrambledin accordance with some embodiments of the invention. In this example,pulses are emitted with a nominal 1 μs period, but with a varying bias,at times 655, 660, . . . , 680. The delay t₀ between emission times 655and 660 is 1 μs, the delay t₁ between emission times 660 and 665 is 1.01μs, the delay t₂ between emission times 665 and 670 is 0.97 μs, and thedelay t₃ between emission times 670 and 675 is 1.02 μs. The pattern thenrepeats, so that the delay t₀ between emission times 675 and 680 is 1μs, etc.

Measured distances over a first interval (interval 0) up to about 150 mis unaffected by the variation of pulse emission times. Measureddistances over a second interval (interval 1) of about 150 m-300 m arebiased by the variation of pulse emission times. Measured distances overa third interval (interval 2) of about 300 m-450 m are biased by thevariation of pulse emission times.

FIG. 6C is a table 685 showing delta distance for each index in a secondinterval in accordance with some embodiments of the invention. For index0, the time difference t₁−t₀=10 ns. For index 1, the time differencet₂−t₁=−40 ns. For index 2, the time difference t₃−t₂=50 ns. For index 3,the time difference t₀−t₃=−20 ns. The time difference for each index inthe table of FIG. 6C is unique. The bias pattern repeats after fourpulses in this example.

FIG. 6D is a table 690 showing delta distance for each index in a thirdinterval in accordance with some embodiments of the invention. For index0, the time difference t₂−t₀=−30 ns. For index 1, the time differencet₃−t₁=10 ns. For index 2, the time difference t₀−t₂=30 ns. For index 3,the time difference t₁−t₃=−10 ns. The time difference for each index inthe table of FIG. 6D is unique. Also, the time difference for each indexin the tables of FIG. 6C and FIG. 6D are different.

FIG. 7 indicates how an interval range is obtained for each of aplurality of intervals in accordance with some embodiments of theinvention. The interval range is determined byInterval Range=(speed of light/2)/emission ratesuch that interval 0 is from 0 m to the first interval range, interval 1is from the first interval range to the second interval range, interval2 is from the second interval range to the third interval range, etc.

FIG. 8A is a timeline showing the result of measurements in a firstinterval (interval 0), using pulses emitted periodically withoutscrambling. In this example, the pulse emission times are spaced at 1μs. Measured distances a are all correct because they are withininterval 0.

FIG. 8B is a timeline showing the result of measurements in a firstinterval (interval 0), using pulses whose emission times are varied inaccordance with some embodiments of the invention. In this example, thepulse emission times are varied as in FIG. 6B. Measured distances a areall correct because they are within interval 0.

FIG. 9A is a timeline showing the result of measurements in a secondinterval (interval 1), using pulses emitted periodically withoutscrambling. In this example, the pulse emission times are spaced at 1μs. Measured distances b are all incorrect because they are withininterval 1; the correct distance should be b+interval 0.

FIG. 9B is a timeline showing the result of measurements in a secondinterval (interval 1), using pulses whose emission times are scrambledin accordance with some embodiments of the invention. In this example,the pulse emission times are spaced as in FIG. 6B. Measured distances bare all incorrect because they are within interval 1, and are biased bythe index values of FIG. 6C.

FIG. 10A is a timeline showing the result of measurements in a thirdinterval (interval 2), using pulses emitted periodically withoutscrambling. In this example, the pulse emission times are spaced at 1μs. Measured distances c are all incorrect because they are withininterval 2; the correct distance should be c+interval 0+interval 1.

FIG. 10B is a timeline showing the result of measurements in a thirdinterval (interval 2), using pulses whose emission times are scrambledin accordance with some embodiments of the invention. In this example,the pulse emission times are spaced as in FIG. 6B. Measured distances care all incorrect because they are within interval 2, and are biased bythe index values of FIG. 6D.

FIG. 11A schematically illustrates distance measurements in a secondinterval (interval 1), using pulses emitted periodically withoutscrambling. Scanning system 1102 scans the face of a building 1104 lyingwithin interval 1. Measurement along path 1110 appears as measureddistance 1115, measurement along path 1120 appears as measured distance1125, measurement along path 1130 appears as measured distance 1135, andmeasurement along path 1140 appears as measured distance 1145. Measureddistances 1115, 1125, 1135 and 1145 are at a nominal range b withininterval 0, while the true distances lie within interval 1.

FIG. 11B schematically illustrates measurements in a second interval,using pulses whose emission times are scrambled in accordance with someembodiments of the invention. Scanning system 1152 scans the face of abuilding 1154 lying within interval 1. Measurement along path 1160appears as measured distance 1165, measurement along path 1170 appearsas measured distance 1175, measurement along path 1180 appears asmeasured distance 1185, and measurement along path 1190 appears asmeasured distance 1195. Measured distances 1165, 1175, 1185 and 1195 areat a nominal range b within interval 0, while the true distances liewithin interval 1, but the measured distances are biased by thevariation of emission times of the respective measurement pulses.

FIG. 12 shows a method of identifying that measurements are in a secondinterval, in accordance with some embodiments of the invention. In thisexample, subtracting consecutive distance measurements recovers theindex values of the table of FIG. 6C for interval 1. The measureddistances which incorrectly appear to be at a nominal range b withininterval 0 can thus be recognized as belonging to interval 1.

FIG. 13 illustrates at 1300 the measurement of distances with pulseswhose emission times are scrambled in accordance with some embodimentsof the invention. Distance meter control unit section 1305 and section1310 access a table of time shifts 1315. A delay or advancecorresponding to the table is applied at 1320 to the nominal pulseemission time so that emission of pulses is controlled at 1325 tointroduce the desired bias pattern to pulses emitted at 1330. Returnpulses received at reception unit 1335 are correlated with emittedpulses to calculate a measured distance and intensity at 1340. Eachmeasured distance is associated at 1345 with an index value derived fromtable 1315, e.g., an index value of table 6C. Each measurement thus hasan associated index value, measured distance and intensity as indicatedat 1350.

FIG. 14 illustrates at 1400 a method of identification of measurementinterval in accordance with some embodiments of the invention. Adistance meter 1405 of a scanning system provides a current measureddistance 1410. Current measured distance 1410 is differenced with aprevious measured distance 1415 at 1420 to obtain a measured deltadistance 1425. Distance meter 1405 increments a pointer to index 1430which retrieves an element from a table 1435 of expected delta distancefor interval 1. Table 1435 is as shown, for example, in FIG. 6C. Theexpected delta distance for interval 1 is differenced with measureddelta distance 1425 at 1440. If the result of differencing at 1440 isdetermined at 1445 to be less than a threshold value, the currentmeasured distance is deemed to be within interval 1. The pattern ofexpected biases of interval 1 is shown at 1450.

Index 1430 also retrieves an element from a table 1455 of expected deltadistance for interval 2. Table 1455 is as shown, for example, in FIG.6D. The expected delta distance for interval 2 is differenced withmeasured delta distance 1425 at 1460. If the result of differencing at1460 is determined at 1465 to be less than a threshold value, thecurrent measured distance is deemed to be within interval 2. The patternof expected biases of interval 2 is shown at 1470, and differs from thepattern 1450 of interval 1.

Index 1430 also retrieves an element from a table 1475 of expected deltadistance for interval n. Table 1475 is comparable to those shown, forexample, in FIG. 6C and FIG. 6D, for a further distance interval. Theexpected delta distance for interval n is differenced with measureddelta distance 1425 at 1480. If the result of differencing at 1480 isdetermined at 1485 to be less than a threshold value, the currentmeasured distance is deemed to be within interval n. The pattern ofexpected biases of interval n is not shown, but differs from the pattern1450 of interval 1 and the pattern 1470 of interval 2.

If the measured delta distance 1425 is not determined at 1445 to bewithin interval 1, or at 1465 to be within interval 2, or at 1485 to bewithin interval n, then it is deemed to be within interval 0.

The method of FIG. 14 can be modified by eliminating table 1475,differencing 1480 and determining 1485, so that identification ofinterval is limited to interval 1, interval 2 and default interval 0.

The method of FIG. 14 can be modified by eliminating tables 1475 and1455, differencing 1480 and 1460, and determination 1485 and 1465, sothat identification of interval is limited to interval 1 and defaultinterval 0.

FIG. 15 shows at 1500 an example of interval ambiguity for the case oftwo intervals. A scanning system 1505 measures at a range of 1510 withininterval 1, but the measured distance is incorrectly at range 1515within interval 0 due to measurement ambiguity.

In some embodiments of the invention, the measured distance at range1515 is determined to be incorrect and is identified as an incorrectmeasurement so that it can be suppressed and thus not appear (e.g., as a“ghost” point at incorrect range) in a point cloud display.

In some embodiments of the invention, the measured distance at range1515 is determined to be incorrect and is identified as belonging tointerval 1 so that it can be displayed correctly in a point clouddisplay.

The methods described above have been found to substantially mitigatethe effect of measurement distance ambiguity by either suppressingincorrect measurements or reassigning incorrect measurements to theirrespective correct range intervals. Further mitigation is possible withthe addition of “point tagging” as will now be described.

FIG. 16 shows at 1600 an overview of point tagging in accordance withsome embodiments of the invention. A point interpreter 1605 carries outtwo stages of tagging. The first stage 1610 performs substantiallyreal-time point tagging 1615 by evaluating whether the current measureddistance fits within a pattern (or “signature”) such as pattern 1450 ofinterval 1 or pattern 1470 of interval 2. The second stage 1620 appliesa rule to assign the current measured distance to an interval based onits relationship to nearby prior and/or subsequent measurements (“tagcontagion”).

FIG. 17 shows point-tagging rules in accordance with some embodiments ofthe invention. In the first stage 1610, a current measured distance (ofpoint p) is tagged as belonging to an interval if the delta distancecorresponds to one of the interval tables. Referring to the example ofFIG. 14, a current measured distance 1410 is assigned to interval 1 ifthe measured delta distance 1425 between the current measured distance1410 of point p and the measured distance 1415 of prior point p−1matches the expected delta distance of table 1435 for the indexassociated with current measured distance 1410. That is, the currentmeasured distance is assigned to interval 1 if the measured deltadistance (p−1, p) conforms to the pattern 1450 (“signature”) defined intable 1435.

As shown in FIG. 17, the second stage 1620 applies further rules suchthat a current measured distance of point p is tagged as belonging to aninterval if: prior measured distance p−1 is missing and subsequentmeasured distance p+1 is tagged as belonging to that interval; bothprior measured distance p−1 and subsequent measured distance p+1 aretagged as belonging to that interval; current measured distance p ismissing and prior measured distance p−1 is tagged as belonging to thatinterval.

Examples of the application of point tagging will now be described.

FIG. 18A shows a scanning scenario 1800 in which a scanning system (notshown) scans vertically downward as shown by arrow 1805 from the sky1810 along 5 the face of a building 1815.

FIG. 18B shows at 1850 an example of point tagging in the scenario ofFIG. 18A in accordance with some embodiments of the invention. When thesky is scanned, no return is detected by distance meter, e.g., nomeasured distance p−1 is obtained at 1855. As the scan reaches the upperface of building 1815, a current measured distance p is obtained at1860. Comparison of current measured distance p with the prior measureddistance p−1 is not possible because prior measured distance p−1 ismissing.

Current measured distance p is retained, but left untagged until asubsequent measured distance p+1 is obtained at 1865. Where comparisonof the difference between measured distance p+1 and measured distance ppermits recognition of an interval pattern and assignment of measureddistance p+1 to an interval (e.g., interval 1), then the rule is appliedto also assign measured distance p to that interval. A single comparisonof measured distance p+1 with measured distance p may not be sufficientto recognize an interval pattern, but where comparison of a subsequentmeasured distance p+2 with measured distance p+1 enables recognition ofthe pattern, then the rule can be applied to assign measured distance pto the same interval.

FIG. 19A shows a scanning scenario 1900 in which a scanning system (notshown) scans vertically downward as shown by arrow 1905 along the faceof a building 1910 and the face of a closer building 1915. Transitionsbetween surfaces lying in different intervals can interfere withrecognition of the pattern of an interval.

FIG. 19B shows at 1950 an example of point tagging in the scenario ofFIG. 19A in accordance with some embodiments of the invention. Wheremeasured distance p−1 at 1955 and measured distance p+1 at 1965 are bothidentified as belonging to an interval (e.g., interval 1), then measureddistance p lying between them at 1960 is assigned to the same interval.

FIG. 20A shows a scanning scenario 2000 in which a scanning system (notshown) scans vertically downward as shown by arrow 2005 along the faceof a building 2010. Some points are missing as indicated at 2015.Missing points can interfere with recognition of the pattern of aninterval.

FIG. 20B shows at 2050 an example of point tagging in the scenario ofFIG. 20A in accordance with some embodiments of the invention. Wheremeasured distance p is missing at 2060 and measured distance p−1 at 2065is identified as belonging to an interval (e.g., interval 1), then themissing measured distance p is assigned to the same interval.

FIG. 21 shows at 2100 an example of tag contagion in accordance withsome embodiments of the invention. The measured distance of a point p istagged as belonging to an interval if it lies between two points taggedas belonging to that interval, which are less distant than some number(e.g., 6) of measurement points. For example, scanline 2105 has a seriesof measured distances missing as indicated by the zeros in the six scanpoint locations 2110 and a series of measured distances missing in thefive scan point locations 2115, and has measured distances present andassigned to interval 1 in the scan point locations 2120 and 2125.Applying the 5 tag contagion rule, the scan point locations 2115 areassigned to interval 1 and the tags of scan point locations 2115 arechanged from 0 to 1.

FIG. 22 is a flow chart of a point tagging method 2200 using tagcontagion in accordance with some embodiments of the invention. Ameasured distance p is selected at 2205 and checked at 2210 for a tagidentifying the interval to which it is assigned. If a tag is found,then at 2215 a check is made whether the difference between scanlineposition of the last measured distance having a tag is greater than oneand less than a selected number of scanline positions away (e.g.,tolerance=6). If yes, then at 2220 each scanline position q within thetolerance from the scanline position of measured distance p is taken inturn and at 2225 assigned a tag associated with the interval of the lastmeasured distance having a tag. When at 2230 the last tagged position isthat of measured distance p, the process returns to select a nextmeasured distance p at 2205. If a measured distance p selected at 2205is not tagged, then the next measured distance p is selected. If at 2215the difference between scanline position of the last measured distancehaving a tag not is greater than one and less than a selected number ofscanline positions away, then the last tagged position is defined asthat of measured distance p.

FIG. 23 describes at 2300 extensions of the inventive concepts. Themethods described above have been found well-suited for walls and othercontinuous surfaces. However, the tag contagion concept can be modifiedwithin the scope of the invention to take into account not only the tagidentifying the interval to which a measured-distance point is assigned,but also the measured distance; in this way, the contagion can berestricted so that nearby measured distances are assigned to the sameinterval as the measured distance under consideration only if thedifference of the measured distance is within a selected thresholddistance.

A further extension of the inventive concepts is to introduce scramblingof the emission times of the measurement pulses not only along each scanline but also across adjacent scan lines, and to use the resultingtwo-dimensional patterns to associate measured distances with rangeintervals.

FIG. 24 illustrates at 2400 a two-dimensional extension in which theemission times of measurement pulses are scrambled in accordance withsome embodiments of the invention. Four scan lines are shown at 2405 inthis example. The scanner system scans lines in a vertical direction asindicated at 2410. Consecutive scan lines are offset in a horizontal(e.g., azimuthal) direction as indicated at 2415. The emission times ofpulses within each scan line are shifted in a repetitive patternindicated by the shifts 1, 2, 3, 4. The emission times of pulses inconsecutive scan lines are shifted in a pattern such that the time shiftof a given measured distance has no neighboring points with the sametime shift, e.g. a point with shift 1 is surrounded by neighboringpoints having shifts 2, 3 and 4, but not 1. The example of FIG. 24 showsmaximization of time shift differences for a signature having fourpossible time shifts in a signature pattern.

FIG. 25 illustrates at 2500 a two-dimensional extension of signaturerecognition by neighborhood selection in accordance with someembodiments of the invention. The four scan lines 2405 of FIG. 24 areshown in this example, with scan lines in the vertical direction 2410offset consecutively in the horizontal direction 2415. In this example,recognition of the pulse emission time signature of a range interval isdetermined using at least one measured distance of the neighborhoodinvolving another scan line. For example, measured distance 2505 havingan emission time shift 3 is compared with one or more neighboringmeasured distances 2510, 2515 of an adjacent scan line having, e.g.,emission time shifts 4 and 1, respectively, for purposes of recognizingthe range interval signature. Measured distance 2505 may also becompared with one or more neighboring measured distances of the samescan line, such as 2520, to identify a two-dimensional signature patternof emission time shifts.

FIG. 26 describes at 2600 an interval assignment policy for atwo-dimensional extension in accordance with some embodiments of theinvention. In this example, each measured distance is compared with atleast one other measured distance in its neighborhood. Each comparisonends with assignment of the evaluated measured distance to a rangeinterval, e.g., interval 1 or interval 0. A strategy of collaboration ofthe interval assignments brings a resulting assignment for the evaluatedmeasured distance.

For example a policy of resulting interval assignment can be one of:assignment to the furthest range interval, assignment to the rangeinterval to which the most measured distance points are assigned, themedian of range interval assignments, and a weighting based on theangular distance of the neighbor from the evaluated measured distancepoint.

FIG. 27 shows a method in accordance with some embodiments of theinvention, of measuring distance from an instrument origin to each of aplurality of points in an environment. Laser pulses are emitted at 2705along a measurement axis at successive displacements about theinstrument origin, the emission time of each emitted laser pulse havinga time shift relative to nominal fixed rate, where the time shiftcorresponds to an index of a repetitive sequential pattern. Receivedpulses are detected at 2710 at respective arrival times. In general,received pulses are detected continuously as the emitted pulses arebeing emitted, each received pulse corresponding to one of the emittedpulses. The received pulses are processed at 2715.

For each received pulse, a current apparent distance is determined at2720 using the arrival time of the received pulse and the emission timeof the latest emitted pulse. At 2725 a measured delta distance betweenthe current apparent distance and a previous apparent distance iscalculated. At 2730, a range interval is assigned by comparing themeasured delta distance with at least one expected delta distance, whereeach expected delta distance is synchronized with the index of thelatest emitted pulse. At 2735, the current apparent distance is definedto be a true measured distance for any received pulse assigned to afirst time interval (e.g., interval 0), and is otherwise defined to be afalse measured distance. The true measured distances are stored at 2740.

In some embodiments, the false measured distances are discarded.

In some embodiments, the false measured distances are identified asinvalid. For example, the false measured distances are flagged asinvalid or are set to a zero value or are set to an invalid value.

In some embodiments, each of a plurality of false measured distances iscorrected to obtain a true measured distance by adjusting for the timeshift and adding an offset corresponding to the assigned range interval.In some embodiments, adjusting for the time shift comprises subtractingan expected delta distance from the current measured distance.

In some embodiments, at least one rule is applied to redefine at leastone true measured distance as a false measured distance. In someembodiments, the rule is based on neighboring measured distances. Insome embodiments the rule is based on whether a prior measured distanceis missing and a subsequent measured distance is assigned to a rangeinterval other than the first range interval. In some embodiments therule is based on whether a prior measured distance and a subsequentmeasured distance is assigned to a range interval. In some embodimentsthe rule is based on a missing measured distance where a subsequentmeasured distance is assigned to a range interval.

In some embodiments, the successive displacements about the instrumentorigin are at least one of angular and linear.

In some embodiments, the repetitive sequential pattern has a fixednumber of time-shift steps, each step associated with an index. In someembodiments, the index corresponds to a unique expected delta distancefor each range interval. In some embodiments, each expected deltadistance corresponds to a time difference between a fixed number ofsteps of the repetitive sequential pattern.

FIG. 28 schematically shows a system 2800 in accordance with someembodiments of the invention, for measuring distance from an instrumentorigin to each of a plurality of points in an environment. System 2800includes a laser source 2805, a reception unit 2810, a system controlunit 2815, and memory 2820. Laser source 2805 is controlled by a lasercontroller 2825 to emit laser pulses along a measurement axis 2830 atsuccessive displacements 2835 about an instrument origin 2840. Theemission time of each emitted laser pulse has a time shift relative tonominal fixed rate, such as in the example of FIG. 6B. The time shiftcorresponds to an index of a repetitive sequential pattern, such as inthe example of FIG. 6C.

Reception unit 2810 detects received pulses at respective arrival times.Control unit 2815 includes a processor 2845 programmed to process eachreceived pulse as in FIG. 27. For each received pulse, processor 2845 isoperative to: determine a current apparent distance using the arrivaltime of the received pulse and the emission time of the latest emittedpulse, calculate a measured delta distance between the current apparentdistance and a previous apparent distance, assign a range interval bycomparing the measured delta distance with at least one expected deltadistance, where each expected delta distance is synchronized with theindex of the latest emitted pulse, define the current apparent distanceto be a true measured distance for any received pulse assigned to afirst time interval, and define the current apparent distance to be afalse measured distance for any received pulse not assigned to the firsttime interval. Memory 2820 is operative to store the true measureddistances.

Programmed processor 2845 is operative to perform one or more additionalfunctions, in accordance with some embodiments of the invention. In someembodiments, processor 2845 is operative to discard the false measureddistances. In some embodiments, processor 2845 is operative to identifythe false measured distances as invalid.

In some embodiments, programmed processor 2845 is operative to correcteach of a plurality of false measured distances to obtain a truemeasured distance by adjusting for the time shift and adding an offsetcorresponding to the assigned range interval. In some embodiments,adjusting for the time shift comprises subtracting an expected deltadistance from the current measured distance.

In some embodiments, programmed processor 2845 is operative to identifya pattern of successive definitions of current apparent distances, andapply a rule to redefine at least one true measured distance as a falsemeasured distance.

In some embodiments, the programmed processor 2845 is operative todetermine the successive definitions along a scan line and/or betweenadjacent scan lines. In some embodiments, the successive displacementsabout the instrument origin are angular and/or linear.

In some embodiments, the programmed processor 2845 is external tocontrol unit 2815. In some embodiments, one or more of the functionsattributed to programmed processor 2845 in the above description arecarried out by one or more other programmed processors within thescanner system and/or external to the scanner system.

In some embodiments, the repetitive sequential pattern has a fixednumber of time-shift steps, each step associated with an index, e.g., asin the example of FIG. 6B and FIG. 6C. In some embodiments, the indexcorresponds to a unique expected delta distance for each range interval.In some embodiments, each expected delta distance corresponds to a timedifference between a fixed number of steps of the repetitive sequentialpattern.

The principle of scrambling the point cloud for distances above theambiguity limit by having an irregular pulse repetition rate has beenhere described with two biases introduced (±0.03 μs equates to ±5 m inthe example of FIG. 6B). Some embodiment of the invention use morecomplex bias patterns. In some embodiments a larger number of fixedbiases is used (e.g., with 3 biases: −5 m, 0 m, +5 m). In someembodiments, random biases in a limited range (e.g., random between −5 mand +5 m) are used. Whatever bias pattern is used, the table of deltadistances is adapted to identify the bias pattern applied to the emittedlaser pulses so that the received pulses can be assigned to theirrespective intervals.

A pattern with more than two biases is used for return pulses comingfrom multiple ambiguity intervals. Measured points in the third interval(interval 2, e.g., for the range 300 m-450 m) would appear unscrambledif a pattern with only two biases were used. In general, to identifypoints that could potentially come from a number of ambiguity intervals,the number of biases is equal to or greater than the number of ambiguityintervals. Pulse power and signal-to-noise ratio of the scanner systemcan limit the number of ambiguity intervals. In some embodiments, ascanner system having a minimum of three biases is capable of measuringover three ambiguity intervals: no ambiguity [0 m-150 m], ambiguityinterval 1 [150 m-300 m] and ambiguity interval 2 [300 m-450 m].

In accordance with some embodiments, measured distances beyond theambiguity limit are scrambled by applying a slight irregularity to thepulse emission rate. For a consistent surface (such as a vertical wallof a building), their relative distances follow a pattern similar to thepattern of the biases of this irregularity. The pattern (a relative“signature”) can be found by analyzing a group of consecutive points ona scan line or a group of neighboring points in a scan. For real-timeimplementation (i.e. at the moment of scanning), it is not realistic toanalyze points from multiple scanlines; thus, consecutive points fromthe same scanline are analyzed. A post-processing stage based onneighboring points in a completed scan can be implemented if a strongerfiltering is required.

Each bias applied to the pulse emission rate results in a correspondingbias in the measured distance. Emission time biases are generated sothey are known and correspond to distance biases. A distance biasespattern can be easily computed based on the selected emission biasespattern. Measured distances are then compared to the distance biasespattern to check if they are correlated. If they are, measured distanceis above the ambiguity interval limit.

In a real world scenario, consecutive measured distances are not equal.That would describe a shape like a perfect sphere centered on thescanner location. So a direct correlation of the measured distances andthe distance biases pattern are easy to recognize is not ideal. For arealistic scenario, the derivative of the measured distances is comparedwith the derivative of the distance biases pattern. Comparison is madebetween the sum of measured distance and a number of previous distances,and the sum of the corresponding distance biases. A threshold is thenused on this comparison to identify points likely to be from distancesabove the ambiguity interval limit.

This comparison allows distance measurements at a range greater than thedistance ambiguity interval to be identified if they describe aconsistent geometry with their neighbors. A set of points not showing acertain consistency (such as trees) will not be completely identified.

The proposed approach is suitable for even longer range, such as amining version of a scanner; mobile (terrestrial) and aerial scanningcould also benefit from it.

Those skilled in the art will realize that the detailed description ofembodiments of the present invention is illustrative only and is notintended to be in any way limiting. Other embodiments of the presentinvention will readily suggest themselves to such skilled persons havingthe benefit of this disclosure.

Reference is made in detail to implementations of the present inventionas illustrated in the accompanying drawings. The same referenceindicators are used throughout the drawings and the following detaileddescription to refer to the same or like parts.

In the interest of clarity, not all of the routine features of theimplementations described herein are shown and described. It will beappreciated that in the development of any such actual implementation,numerous implementation-specific decisions must be made to achieve thedeveloper's specific goals, such as compliance with application- andbusiness-related constraints, and that these specific goals will varyfrom one implementation to another and from one developer to another.Moreover, it will be appreciated that such a development effort might becomplex and time consuming, but would nevertheless be a routineundertaking of engineering for those of ordinary skill in the art havingthe benefit of this disclosure.

In accordance with embodiments of the present invention, the components,process steps and/or data structures may be implemented using varioustypes of operating systems (OS), computer platforms, firmware, computerprograms, computer languages and/or general-purpose machines. Portionsof the methods can be run as a programmed process running on processingcircuitry. The processing circuitry can take the form of numerouscombinations of processors and operating systems, or a stand-alonedevice. The processes can be implemented as instructions executed bysuch hardware, by hardware alone, or by any combination thereof. Thesoftware may be stored on a program storage device readable by amachine. Computational elements can be readily implemented using anobject-oriented programming language such that each required element isinstantiated as needed.

Those of skill in the art will recognize that devices of a lessgeneral-purpose nature, such as hardwired devices, field programmablelogic devices (FPLDs), including field programmable gate arrays (FPGAs)and complex programmable logic devices (CPLDs), application specificintegrated circuits (ASICs), or the like, may also be used withoutdeparting from the scope and spirit of the inventive concepts disclosedherein.

In accordance with an embodiment of the present invention, the methodsmay be implemented in part on a data processing computer such as aportable computing device, personal computer, workstation computer,mainframe computer, or high-performance server running an operatingsystem such as Microsoft® Windows®, available from Microsoft Corporationof Redmond, Wash., or various versions of the Unix operating system suchas Linux available from a number of vendors, or a version of the Androidoperating system. The methods may also be implemented on amultiple-processor system, or in a computing environment includingvarious peripherals such as input devices, output devices, displays,pointing devices, memories, storage devices, media interfaces fortransferring data to and from the processor(s), and the like. Such acomputer system or computing environment may be networked locally, orover the Internet.

Some or all of the above-described methods and their embodiments may beimplemented in part by execution of a computer program. The computerprogram may be loaded on an apparatus as described above. Therefore, theinvention also relates to a computer program, which, when carried out onan apparatus performs portions of any one of the above above-describedmethods and their embodiments.

The invention also relates to a computer-readable medium or acomputer-program product including the above-mentioned computer program.The computer-readable medium or computer-program product may forinstance be a magnetic tape, an optical memory disk, a magnetic disk, amagneto-optical disk, a CD ROM, a DVD, a CD, a flash memory unit or thelike, wherein the computer program is permanently or temporarily stored.The invention also relates to a computer-readable medium (or to acomputer-program product) having computer-executable instructions forcarrying out any one of the methods of the invention.

The invention also relates to a firmware update adapted to be installedon apparatus already in the field, i.e. a computer program which isdelivered to the field as a computer program product. This applies toeach of the above-described methods and apparatuses.

Although the present invention has been described on the basis ofdetailed examples, the detailed examples only serve to provide theskilled person with a better understanding, and are not intended tolimit the scope of the invention. The scope of the invention is muchrather defined by the appended claims.

The invention claimed is:
 1. A method of measuring distance from aninstrument origin to each of a plurality of points in an environmentcomprising: emitting laser pulses along a measurement axis at successivedisplacements about the instrument origin, an emission time of eachemitted laser pulse having a time shift relative to nominal fixed rate,where the time shift corresponds to an index of a repetitive sequentialpattern; detecting received pulses at respective arrival times; for eachreceived pulse: determining a current apparent distance using thearrival time of a received pulse and the emission time of a latestemitted pulse; calculating a measured delta distance between the currentapparent distance and a previous apparent distance; assigning a rangeinterval by comparing the measured delta distance with at least oneexpected delta distance, where each expected delta distance issynchronized with the index of the repetitive sequential pattern of thelatest emitted pulse; defining the current apparent distance to be atrue measured distance for any received pulse assigned to a first rangeinterval; and defining the current apparent distance to be a falsemeasured distance for any received pulse not assigned to the first rangeinterval; storing each true measured distance; and for each falsemeasured distance: determining the false measured distance satisfies apoint tagging criteria; thereafter, assigning the first range intervalassociated with the true measured distance to the false measureddistance; and storing the false measured distance assigned to the firstrange interval with the true measured distances.
 2. The method of claim1 wherein the point tagging criteria comprises determining the falsemeasured distance precedes at least one true measured distance.
 3. Themethod of claim 1 wherein the point tagging criteria comprisesdetermining the false measured distance was received subsequent to afirst true measured distance.
 4. The method of claim 3 wherein the pointtagging criteria further comprises determining the false measureddistance precedes a second true measured distance.
 5. The method ofclaim 1 wherein the point tagging criteria comprises: determining thefalse measured distance is one false measured distance of a plurality ofconsecutive false measured distances; determining the plurality ofconsecutive false measured distances is preceded by a first truemeasurement and succeeded by a second true measurement; determining atotal number of false measured distances in the plurality of consecutivefalse measured distances; and determining the total number of falsemeasured distances is less than a threshold number of false measureddistances.
 6. The method of claim 1 wherein the point tagging criteriacomprises determining the false measured distance is within a thresholddisplacement along the measurement axis.
 7. The method of claim 1further comprising: storing the current apparent distance for eachreceived pulse in one or more scan lines; wherein each scan linecorresponds to a first scan direction, and wherein adjacent scan linescorrespond to a second scan direction; and determining the falsemeasured distance in a first scan line satisfies the point taggingcriteria using an adjacent scan line.
 8. The method of claim 1, whereinthe repetitive sequential pattern has a fixed number of time-shiftsteps, each step associated with an index.
 9. The method of claim 1,wherein the index corresponds to a unique expected delta distance foreach range interval.
 10. The method of claim 1, wherein each expecteddelta distance corresponds to a time difference between a fixed numberof steps of the repetitive sequential pattern.
 11. Apparatus formeasuring distance from an instrument origin to each of a plurality ofpoints in an environment, comprising: a laser source to emit laserpulses along a measurement axis at successive displacements about theinstrument origin, an emission time of each emitted laser pulse having atime shift relative to nominal fixed rate, where the time shiftcorresponds to an index of a repetitive sequential pattern, a receptionunit to detect received pulses at respective arrival times, at least onecontrol unit operative, for each received pulse, to: determine a currentapparent distance using an arrival time of the received pulse and anemission time of a latest emitted pulse, calculate a measured deltadistance between the current apparent distance and a previous apparentdistance, assign a range interval by comparing the measured deltadistance with at least one expected delta distance, where each expecteddelta distance is synchronized with the index of the repetitivesequential pattern of the latest emitted pulse, define the currentapparent distance to be a true measured distance for any received pulseassigned to a first range interval, define the current apparent distanceto be a false measured distance for any received pulse not assigned tothe first range interval, wherein for each false measured distance thecontrol unit is operative to: determine the false measured distancesatisfies a point tagging criteria; and thereafter, assign the firstrange interval associated with the true measured distance to the falsemeasured distance; and a memory operative to store the true measureddistances and the false measured distances assigned to the first rangeinterval.
 12. The apparatus of claim 11 wherein the point taggingcriteria comprises determining the false measured distance precedes atleast one true measured distance.
 13. The apparatus of claim 11 whereinthe point tagging criteria comprises determining the false measureddistance was received subsequent to a first true measured distance. 14.The apparatus of claim 13 wherein the point tagging criteria furthercomprises determining the false measured distance precedes a second truemeasured distance.
 15. The apparatus of claim 11 wherein the pointtagging criteria comprises: determining the false measured distance isone false measured distance of a plurality of consecutive false measureddistances; determining the plurality of consecutive false measureddistances is preceded by a first true measurement and succeeded by asecond true measurement; determining a total number of false measureddistances in the plurality of consecutive false measured distances; anddetermining the total number of false measured distances is less than athreshold number of false measured distances.
 16. The apparatus of claim11 wherein the point tagging criteria comprises determining the falsemeasured distance is within a threshold displacement along themeasurement axis.
 17. The apparatus of claim 11 further comprising:storing the current apparent distance for each received pulse in one ormore scan lines; wherein each scan line corresponds to a first scandirection, and wherein adjacent scan lines correspond to a second scandirection; and determining the false measured distance in a first scanline satisfies the point tagging criteria using an adjacent scan line.18. The apparatus of claim 11 wherein the repetitive sequential patternhas a fixed number of time-shift steps, each step associated with anindex.
 19. The apparatus of claim 11, wherein the index corresponds to aunique expected delta distance for each range interval.
 20. A method ofmeasuring distance from an instrument origin to each of a plurality ofpoints in an environment comprising: emitting laser pulses along ameasurement axis, wherein an emission time of each emitted laser pulsehas a time shift relative to nominal fixed rate; detecting receivedpulses at respective times; determining a measured distance for eachreceived pulse; storing the measured distance in a scan line position;and for each measured distance: determining the measured distance iswithin a threshold number of scan line positions of a previous measureddistance assigned to a range interval, wherein the range interval isassigned by: comparing successively measured distances to obtain deltadistances; and comparing the obtained delta distances with expecteddelta distances; and thereafter, assigning the measured distance to therange interval.